1. Field of the Invention
This invention relates to a printed circuit board having high shock resistance and an electronic device including the same.
2. Description of the Related Art
Conventionally, a printed circuit board for mounting electronic parts is composed of a glass-epoxy substrate including laminated sheets, each of which is made of a glass cloth impregnated with epoxy-prepreg. This kind of circuit board has been generally used with high reliability for mounting parts such as QFP having lead terminals, parts such as a chip resistor having a relatively large terminal distance, and the like.
Recently, however, a mounting density is further increased and the terminal distances of the mounted parts are prominently decreased. This requires a reduced distance between electrodes of a board for holding the parts. In addition, in a device complying with the high mounting density requirement for a CSP (Chip Size Package), a MCM (Multi Chip Module), a flip chip, and the like, there is a tendency that the distances between electrodes of the board and mounted part bodies are decreased. This enhances adverse effects by thermal stress and mechanical stress which are produced around the electrodes of the board after the parts are mounted. Especially, when the printed circuit board uses bisphenol family or novolak family epoxy resin, phenol resin or acrylic family resin as a matrix resin for a resin layer, the resin layer of the board and soldering portions easily can have cracks therein by drop shock or thermal shock.
JP-B2-5-46998 proposes a method for preventing the cracks by forming a low elastic modulus polyimide resin layer not including fillers for enforcement. This method is, however, complicated because it must employ two separate steps for forming two kinds of insulation layers including and not including the fillers. In this method, using polyimide resin is disadvantageous to its cost.
JP-A-5-136575 proposes to use a resin material including epoxy resin to which polyvinyl butyral that is thermoplastic resin is added; however, the document does not consider shock resistance property. In addition, polyvinyl butyral itself is disadvantageous in thermal resistance and moisture resistance. If the content of polyvinyl butyral is increased to realize a sufficiently low elastic modulus, leak deficiencies or deformation of the board may occur under harsh usage environment or manufacturing conditions.
Conventionally, various kinds of fillers are mixed with matrix resin of an insulation resin material to increase the strength and to reduce the elastic modulus of the material. For example, when low elastic modulus fillers made of rubber is mixed with the matrix resin to reduce the elastic modulus, thermal shock resistance may be improved; however, it is difficult to improve the mechanical shock resistance. This is because mechanical shock is usually accompanied by relatively large and quick deformation, and accordingly separation and cracks are produced at interfaces between the fillers and the matrix resin. Thus, mixing fillers is insufficient to provide sufficient reliability to a printed circuit board complying with the high mounting density requirement.